The invention applies to the field of hydrodynamics, and relates to the use of cavities to reduce the drag of ships, submarines, torpedoes, hydrofoils, surface-piercing struts, propellers, control surfaces, fairings, and in general, to any underwater surface.
Vapor filled cavities for reducing hydrofoil drag were patented by Tulin (U.S. Pat. No. 3,065,723) and Wennegal (U.S. Pat. No. 3,044,432); the latter patent included leading and trailing edge flaps. Low drag base-vented and side-vented hydrofoil concepts were patented a year later by Lang (U.S. Pat. No. 3,109,495). A low-drag submarine was patented by Lee about the same time wherein the speed was so high that a vapor-filled cavity is formed between an adjustable nose cone and a tail section (U.S. Pat. No. 3,041,992). However, that concept would not work because the submarine is in a constant-pressure vapor cavity which would not provide any lift to support the massive weight of a submarine. Control fins are shown, but these would be far too small to support submarine weight, and no teaching was included to use the fins in that manner. A different means for reducing torpedo drag was patented by Eichenberger in which a gas film was used to cover the surface of a torpedo in which the gas film is so thin that it would sustain the outside pressure changes due to depth and provide displacement lift to support the torpedo weight (U.S. Pat. Nos. 3,016,865 and 3,075,489). However, that concept appears to be complex, and would require accurately-machined surfaces. A different means for reducing torpedo drag by using a gas-filled cavity was patented by Lang in which lift is provided by nosepieces and tailpieces to support torpedo weight (U.S. Pat. No. 3,205,846). Claims in U.S. Pat. No. 3,504,649 include a provision for ejecting upper and lower sheets to reduce frictional drag on displacement hydrofoils; also included were air removal means. However, that patent suggests that the air supplied to the upper and lower surfaces comes from the same plenum, and no mention is made that these air sheets are at different pressures in order to support the vehicle weight. If both air sheets are at the same pressure, one of the air sheets would break up into individual bubbles and lose its ability to reduce drag. Means to reduce the drag on the lower surface of boats was patented by Baldwin (U.S. Pat. No. 1,656,411). That concept is somewhat like the modern SES (surface effect ship) concept. More recent ways to reduce the drag on boat bottoms have been patented by Burg (U.S. Pat. Nos. 5,176,095 and 5,415,120). A recent patent shows how drag can be reduced on hydrofoils by using a special cross-sectional shape which operates either fully wetted, supercavitating or base-cavitated depending upon speed (U.S. Pat. No. 5,601,047).
Marine vehicles are categorized as either displacement craft or dynamic-lift craft. Displacement craft derive their lift from buoyancy (displacement). Dynamic-lift craft derive their lift dynamically, such as by hydrofoils or planing surfaces. The drag of displacement craft is primarily frictional and wavemaking. The drag of dynamic-lift craft is primarily frictional and induced (drag induced by lift). In high-speed craft of either type, frictional drag is normally more than half the total drag. It is important to reduce frictional drag, although all types of drag are reduced to achieve a large lift-to-drag (L/D) ratio. The following are dynamic-lift: hydrofoil ships, air cushion vehicles (ACV), seaplanes, wing-in-ground effect (WIG) craft, planing hydrofoil ships, surface effect ships (SES) and ram wing craft. Displacement craft include: slender monohull ships, catamaran ships, SWATH (Small Waterplane Area Twin Hull) and displacement hydrofoil ships.
The need for reduction in frictional drag has long existed. Means for reducing frictional drag include laminarization, air cavities and air films, riblets, magnetohydrodynamics, microbubble ejection, polymer ejection and moving walls.
The invention provides an air-cavity drag-reducing system, larger-than-normal sweepback on hydrofoils and struts, and control of hydrofoil and strut dynamic forces by controlling their air cavities.
Drag and motion are reduced by factors of 6 or more compared with existing vehicles to transit 10,000 miles at 100 knots without refueling, and to limit vertical accelerations in 30-ft waves to around 0.1 g when either transiting or loitering offshore.
The new vehicles are built using conventional materials, machinery and manufacturing methods.
The benefits of these new vehicles include greatly reduced fuel consumption, longer range, higher speed and greatly improved seakindliness compared with state-of-the-art vehicles.
The new ships deliver a sizable military force almost anywhere in the world within 100 hours. That capability is most important in regions where airfields are unavailable. If immediate action is not required, then these new vehicles have the ability to loiter offshore, show a military presence and sustain troops for one month without resupply. Other military applications include missile launchers, V/STOL or conventional aircraft carriers, arsenal ships and patrol vessels.
Commercial applications include cost-effective ships for rapid delivery of perishable or high-value cargo, fast ferries, commercial fishing vessels and recreational craft. Those applications may be scaled down in size and speed.
The ship hull is supported above water by seven struts attached to a horizontal V-shaped hydrofoil unit which is swept back 70 degrees. The hydrofoil unit has a span of 261 ft, is optimized to operate at a depth of 22 feet, and consists of three independent, adjacent hydrofoils. The propulsion system uses four ducted air propellers, similar to those used on hovercraft. Alternative propulsors are underwater superventilating propellers and pumpjets. The power plants are either gas turbines or diesel engines, and provide the required 137,000 shp to cruise at 100 knots. To meet the range requirement, drag is reduced on the hydrofoils and struts by covering most of their underwater surfaces with air cavities. The struts and hydrofoils are automatically controlled for maintaining near-constant lift and sideforce to provide a near-level ride in up to 30-ft waves. Maneuvering is achieved by banking the ship into turns. Emergency turns and stops are augmented by lowering drag plates into the water.
The hydrofoil hull has a beam of 100 ft, and an at-rest draft of 10 ft with hydrofoils retracted. In a preferred embodiment, the three hydrofoil sections retract rearward and upward. The hydrofoils are retracted when offloading onto a beach, when operating in shallow water or harbors, or when transiting the Suez Canal. Also, the hydrofoils automatically retract in a collision with an underwater obstacle. Alternative hydrofoils not only retract but fold against the hull to clear the Panama Canal. Offloading onto beaches with slopes as shallow as two degrees is done through the stern via four 20-ft-wide retractable ramps which are lowered after trimming the ship bow down one degree, and backing into shore. When loitering, the hydrofoils remain below water to help reduce motion, and air bags are attached to the hydrofoils to further reduce motion.
Vehicle weight (i.e., lift, L) is fixed. Reducing drag, D, is identical to achieving a high lift-to-drag ratio, L/D, a nondimensional parameter useful in comparing different kinds of vehicles.
All sources of drag are reduced together with frictional drag to satisfy speed and range requirements. Induced drag is reduced by reducing lift per unit span. Wavemaking drag is reduced by reducing the beam/length ratio of ship hulls, by submerging hulls, or by strategically positioning multiple bodies to reduce the overall wavemaking drag.
To minimize seasickness and permit near-normal working conditions, accelerations are kept below around 0.1 g, and roll and pitch angles are kept below around 10 degrees. Higher accelerations are permissible for short periods, and the requirements are relaxed somewhat for unusual storm conditions. Few ship types can reach 100 knots, and no known ship types are able to meet the motion requirements at even moderate speeds in 30-ft waves. In ships or other vehicles following the wave surface at 100 knots, vertical accelerations greatly exceed the desired limit. The trajectory of a vehicle must be nearly level. To provide a near-level ride, some type of automatic control, or passive stabilization, system is used. Automatic control involves sensors, computers and controls. Passive stabilization includes the use of a small waterplane area, damping vertical motion, and/or mechanically isolating passengers from ship motion. Automatic control best satisfies motion requirements.
Worldwide, the significant wave height (average of the ⅓ highest waves) does not exceed 5 m (16.4 ft) for 98.4% of the time. In the North Atlantic, that significant wave height is not exceeded 97.5% of the time. Therefore, a significant wave height of 5 meters appears to be a reasonable number for the design wave height. In such sea states, waves can occasionally be as high as 30 ft (9.14 meters). The vehicle needs to limit both vertical and transverse accelerations to 0.1 g in up to 30-ft waves.
Ship motions are caused by the orbital velocities of water particles in waves. A water particle at the surface of a wave follows a near-circular path whose diameter is the wave height. Although the speed of such a particle is nearly constant, its velocity vector migrates through 360 degrees during one period. Water particles beneath the surface follow a similar near-circular path, but their path diameters reduce exponentially with depth.
Waves affect ship motion by changing the following four variables: 1) surface elevation, 2) angle of attack, 3) angle of yaw, and 4) forward speed. A ship designed to travel without significant vertical or sideward acceleration through waves must overcome the change in force produced by changes in these four variables.
The new hydrofoil ship ideally follows a level trajectory. The lift on its hydrofoils remains nearly constant and independent of wave-induced effects, such as angle of attack. The hydrofoils are controlled to cancel the lift change otherwise produced by a change in angle of attack. To control a hydrofoil, it is necessary to know the change in angle of attack induced by a wave.
The conclusions drawn from this wave analysis are:
a) a 100-knot ship must follow a relatively-straight trajectory, independent of waves,
b) angles of attack in pitch and yaw induced by waves are small but significant,
c) changes in velocity induced by waves are small but significant, and
d) all wave-induced effects significantly reduce with depth, especially for shorter waves.
The ability to reach speed can be a critical problem for high-performance, dynamic-lift craft. A ship capable of operating at 100 knots strongly depends upon drag reduction. The ship might not be able to takeoff, or reach speed. Therefore, ship power requirements are explored at takeoff and other speeds to determine if a ship has sufficient power to reach design speed.
State-of-the-art propulsion systems are used for the vehicles. Diesel engines provide the lowest fuel consumption, and cost less than gas turbines. Alternatively, gas turbines are much lighter and smaller than diesels. Either type is acceptable for the vehicles. At 100 knots, conventional propellers cavitate. Supercavitating propellers, superventilating propellers, water jets or ducted air propellers are employed. The drive system is either mechanical, electrical, or hydraulic. Mechanical systems are the most efficient; electric systems provide good design flexibility, but are heavy; and hydraulic systems are smaller and lighter than the others, but lack efficiency.
The maximum beam permitted by the Panama Canal is 106 ft, which provides a clearance of one foot on each side. Typical 10,000 ton ships can clear this canal. However, the span of hydrofoils on hydrofoil ships greatly exceed this beam limit. Folding of the hydrofoils is needed. On the other hand, the Suez Canal is 390 ft wide at the surface and 118 ft wide at the bottom where the depth is 46 ft. The hydrofoils do not require folding, but require retracting.
2,000 tons of equipment and 500 troops are to be offloaded onto an unimproved beach without the use of support vehicles. If the transport vehicle hull has a shallow draft, then offloading is accomplished by trimming the vessel, and offloading directly onto a beach. Alternatively, floating bridges, or other devices, are provided. It is necessary to clear mines and secure the local region before offloading. Advance information on beach contour and local bottom profiles is necessary.
All candidate ships are able to loiter offshore for a month without exceeding vertical accelerations of around 0.1 g. The one-month period provides time for replenishment. Some vehicle types like SWATH ships inherently have low motions at rest. For others, motion-reduction means is employed.
Collisions with other craft and with submerged objects at 100 knots must be avoided. Chances of such collisions are minimized by using state-of-the-art sensors, direct observation, and a global positioning system in conjunction with accurate maps, real-time satellite data and other information on the location of other craft and underwater obstacles. Collisions with large sea animals are rare, but are avoided by transmitting high-intensity underwater sound beams to deter sea animals and drive them away from the vehicle""s path. To help avoid collisions, it is necessary to maneuver quickly and/or reverse thrust. The probability of collisions is minimized by minimizing ship beam and draft. Damage in the case of collisions is minimized by compartmentalization, energy absorbers, and/or retracting or releasing vulnerable appendages such as hydrofoils.
If ship drag is reduced by a large factor, then current ship structure and outfitting are used, and still satisfy the design conditions. Alternatively, very light-weight structural materials, vehicle components, and ship systems are used to carry enough fuel for the otherwise large propulsion systems; however, ship costs greatly increase. Therefore, drag reduction is extremely important in reducing ship cost.
In any case, weight reduction is beneficial unless the increased cost overrides the advantages of reduced weight. Ship structural weight is reduced significantly by using aluminum instead of steel; however, ship cost increases, Steel is currently favored in this size range. Alternatively, new structural concepts and outfitting concepts reduce weight without increasing cost.
The invention reduces drag on the underwater surfaces of marine vehicles by covering the surfaces with gas-filled cavities. The underwater surfaces include hydrofoils, struts, boat and ship hulls, pontoons, underwater bodies, fins, rudders, fairings, protuberances, submarine sails and propulsors. In most cases, air is the preferred gas. As used herein, the word xe2x80x9cairxe2x80x9d is synonymous with the word xe2x80x9cgasxe2x80x9d, and xe2x80x9ccavityxe2x80x9d is synonymous with xe2x80x9cgas-filled cavityxe2x80x9d.
The types of marine vehicles which benefit from this invention include those which operate at the water surface, under the water, and in the air. Examples of surface vehicles are hydrofoil craft, monohulls, catamarans, SWATH (small waterplane area twin hull) craft, and SES (surface-effect ships). Underwater craft include submarines and torpedoes. Air vehicles include water-based WIG (wing-in-ground-effect) vehicles and seaplanes.
The drag-reduction invention applies to combinations of vehicles, propulsion systems and underwater surfaces, wherein at least one of them incorporates at least one wetted nosepiece. At least one gas-filled cavity is formed behind each nosepiece to cover an underwater surface. The nosepieces have controls to control the shapes of said cavities.
In most cases, the cavities are formed by ejecting air from near the end of each nosepiece. Air is ejected at a speed and direction which are close to those of the water at the local cavity wall. Air optionally is ejected at one or more stations downstream of said nosepiece.
Fences are attached to underwater surfaces to separate adjacent cavities. Fences include a wetted region on said underwater surface, a wetted strip of material attached to said surface or a dynamic curtain of moving water, gas or other fluid.
The local cavity thickness is the distance between an underlying surface and the local cavity wall. Local cavity thickness is controlled so that the local cavity air speed is approximately equal to the local water speed at the cavity wall. Local cavity thickness is a function of the airflow rate and includes the local thickness of the air boundary layer which builds up along the underlying surface.
Tailpieces provide air removal. In most cases, one or more tailpieces are used to smoothly close cavities and redirect the water flow back to the free-stream direction. Air is optionally removed from a cavity by an air-removal device which preferably is located near the forward part of a tailpiece, and is attached to the tailpiece.
Air is recycled. Air which is removed from a cavity is recycled by pumps which re-energize the air so that it can be re-injected into a cavity. Air is removed at some station along a cavity, re-energized with a pump and re-injected at or near that station.
Nosepieces have controls. A nosepiece control for a hydrofoil or strut includes a flap located on each side of the nosepiece which pivots outward about a spanwise axis positioned near the leading edge. A spanwise air plenum, or one in each flap, supplies air to a slot-like nozzle located at the flap trailing edge such that a curtain of air is ejected tangential to the trailing edge of the flap, and at a speed close to the local water speed. Moving a flap outward on one side enlarges the cavity on that side.
Lift control is provided by controlling cavity thickness. When a nosepiece flap is used to increase cavity thickness on one side of a hydrofoil or strut, the camber of the virtual hydrofoil is increased on that side, resulting in an increase of dynamic lift on that side. By xe2x80x9cvirtualxe2x80x9d is meant the hydrofoil-like shape, consisting of the combination of wetted surface and cavity, which hydrodynamically acts much like a solid hydrofoil surface. Virtual hydrofoil shape, and its associated lift, are changed by varying the cavity thickness on one side or the other of a hydrofoil.
Tailpiece control may be employed. When a tailpiece is used, and when the size or shape of a cavity is changed by the nosepiece, it may be desirable to change the shape of the tailpiece to more-smoothly close the cavity and to conform with changes in camber. One tailpiece control is an outwardly-movable flap which pivots about a transverse axis located near the trailing edge. To seal the gap when the flap is moved outward, a flexible or a segmented air removal means is placed between the flap leading edge and the fixed underlying surface. A preferred tailpiece control is to simply rotate the tailpiece unit like a single flap which pivots about its leading edge.
Cavity length is controlled. For a given nosepiece flap angle, the rate at which air is removed from a cavity affects cavity length. Some air entrains naturally out of the end of a cavity and forms a bubble-filled wake. In cases where a tailpiece is used, it is important to control cavity length so that the cavity closes within a limited region near the front of the tailpiece. The primary control of cavity thickness is deflection of a nosepiece flap. The primary control of cavity length is limitation of the airflow rate, thereby limiting cavity length. Cavity length increases when the airflow rate increases. A better and faster means for controlling cavity length is to provide an airflow rate which exceeds the natural air entrainment rate, and then to remove the excess air near the end of the cavity. By doing that, cavity length can be controlled fast enough to compensate for changes caused by waves, maneuvering and speed changes. One way to control cavity length is to position the air removal means to cover a limited region at cavity closure, to provide air removal which removes only air and (ideally) not water, and to provide a suction to remove air at a rate faster than the ejected airflow rate. Consequently, if the air removal means is covered with water, then the air removal rate is zero and the cavity will grow longer. Alternatively, if the air removal means is fully covered with air, then the air removal rate exceeds the air supply rate, and the cavity will grow shorter. Air removal devices include a fine-mesh screen, a porous material, or a slot. If water passes through the air removal means, water is separated from the air and pumped outside.
Control sensors are employed. As part of an automatic control system, sensors are used to determine system inputs, such as location and/or thickness of cavities, flow angle approaching the nosepiece, vehicle speed, vehicle accelerations, vehicle turn rate, local component depth, and wave effects.
Hydrofoils are three-dimensional. The upper and lower cavities on hydrofoils are separated by wetted nosepieces and wetted tailpieces. The cavities must also be separated by fences at the ends of each hydrofoil, unless the wetted nosepieces and tailpieces join at the ends. To reduce the drag of a superventilated hydrofoil where the upper surface cavity extends past the trailing edge, the lower surface is covered with a higher-pressure cavity which terminates on an underside tailpiece section. When not superventilated, each upper and lower cavity terminates near the front of a tailpiece. The cavities are controlled as described above.
The hydrofoils employ fences. Because hydrofoil craft preferably bank in turns to eliminate sideforce, the depth along a hydrofoil span varies, causing a variation in cavity pressure along the span. This variation in pressure causes air to flow spanwise from a higher pressure region into a lower pressure region. This spanwise flow is reduced by placing fences along the span which lie nearly parallel to the oncoming water flow.
The invention provides strut drag reduction. To reduce drag on surface piercing struts, a separate cavity is formed on each side behind a wetted nosepiece. Drag in the upper regions of struts is reduced the most if tailpieces are not used, and the two cavities are permitted to join and extend well beyond the trailing edge. In the lower strut regions, tailpieces are used to minimize drag. The cavities are preferably at atmospheric pressure. Air can either be ducted from the atmosphere and ejected through slots to fill cavities, or left to entrain naturally from the atmosphere. To minimize drag, the nosepiece shape, and cavity shape and underlying strut shape vary with depth. Segmented nosepieces can be controlled as a function of depth. At the junctures between struts and hydrofoils or underwater bodies, fences separate any adjacent cavities.
The invention provides hull drag reduction. To reduce drag on ship hulls, such as a catamaran hull, separate cavities are formed on each side of each hull behind wetted nosepieces, much like in the upper region of struts. Drag is generally minimized if tailpieces are not used. The cavities are created to extend behind the hull. The cavities are preferably operated near atmospheric pressure. Air is pulled from the atmosphere to fill the cavities. Because of wave action and maneuvering, additional air ejection slots are placed along each side of the hull. Since cavity convexity increases with depth, the nosepiece width and/or length increases with depth. Because side cavities provide no vertical lift, the hull bottoms are separated from these cavities and operate at close to depth pressure. To minimize drag on a hull bottom, a cavity covers most of the bottom. Fences isolate the bottom cavity from the side cavities. Air is pumped into the bottom cavity from the atmosphere. Pump power is minimized by removing the air near the trailing edge of the bottom surface and recycling it. Dynamic lift and its associated drag are eliminated or minimized if the displacement of the cavities and underwater surfaces is about equal to vessel displacement.
The invention provides submerged bodies with fences. Drag is reduced on submerged bodies in a manner similar to reducing drag on hydrofoils. Lift to support vehicle weight is preferably provided by covering the upper half of a body with a low-pressure cavity and covering the lower half with a high-pressure cavity. The two cavities are separated by a horizontal fence. To eliminate dynamic lift, the displacement of the combined cavities and wetted surfaces must equal vehicle weight. Nosepieces and tailpieces for underwater bodies are similar in function to those of hydrofoils, except that they are three-dimensional in shape. Nosepiece flaps move outward and are made from overlapping segments or from a flexible material. An alternative nosepiece control is a clamshell-like nosepiece flap which pivots outward about a transverse horizontal axis located near the nose. Appropriate changes are made in body shape to maintain cavities when the flaps are deflected. To generate the necessary centripetal force for turning, one option is to roll the body several degrees and reduce the upper cavity pressure. Another option is to add a pair of fences positioned at 90 degrees to the horizontal fences, and vary cavity pressure in each quadrant to control sideforce and lift.
Cavities drive air turbines. To generate power, an air turbine is positioned in the duct leading from the atmosphere to any cavity which has lower than atmospheric pressure. Power is transferred to an air pump or to the general ship power system either electrically, hydraulically or mechanically such as by connecting turbine and pump shafts with appropriate gearing.
The invention minimizes the effects of waves. Waves produce local changes in depth, speed and flow angles. These local changes affect the dynamic forces generated on hydrofoils, struts and underwater bodies. To minimize these local force changes, nosepiece flap angles are automatically controlled, together with airflow rates and tailpiece flap angles. By positioning hydrofoils or bodies to operate at greater depths, wave effects are significantly reduced. They reduce exponentially with depth. Another way to reduce effects is to sweep back hydrofoils and struts. The effects of waves reduce with the cosine of the sweep angle. Wave effects are reduced by half with a sweep angle of 60 degrees, and reduced by a factor of three with a sweep angle of 70.5 degrees. Also, wave-induced flow angles reduce inversely with vehicle speed, so higher speeds are advantageous. If wave effects are small enough, automatic control may not be needed to attain acceptable seakindliness.
The invention applies to different kinds of vehicles. Hydrofoil ships may use swept-back hydrofoils, added fences, added struts, etc. Catamaran ships use drag-reduced hulls, no tailpieces, segmented nosepieces whose segments are independently controllable, and multiple flaps with air ejection slots located along each side of each hull, and optional air layer along the bottom with fences separating the side and bottom cavities, and optional control fins. SWATH ships have twin-bulb lower hulls with two air ejection slots on each to cover the speed range, retractable struts, strut drag reduction, control fins, etc. WIG hulls and seaplane hulls use air ejection along the bottom of the hulls during takeoff and landing, and optional air ejection along the hull sides.
Applications include tanker ships, and many kinds of commercial and pleasure craft.